Journal of Sciences, Islamic Republic of Iran 19(3): 223-235 (2008) http://jsciences.ut.ac.ir University of Tehran, ISSN 1016-1104

Post-Collisional Plio-Pleistocene Adakitic Volcanism in Centeral Iranian Volcanic Belt:Geochemical and Geodynamic Implications

G. Ghadami,1,* A.M. Shahre Babaki,1 and M. Mortazavi2

1Department of Geology, Faculty of Science, University of Shahid Bahonar, 76175-133 Kerman, Islamic Republic of Iran 2Department of Geology, Faculty of Science University of Hormozgan, 3995 Bandar Abass, Islamic Republic of Iran

Abstract In the Central IranianVolcanic Belt (CIVB), north-west of Shahre-Babak, in the area of Javazm, Dehaj and khabr, about 60 subvolcanic porphyritic dacitic to rhyodacitic domes (1-10 km2) are intruded into a variety of rock sequences from Mesozoic to Early Miocene in age. These rocks are a part of Dehaj-Sardoieh belt. The CIVB contains intrusive and extrusive rocks of Cretaceous-Quaternary age. Geochemical data indicate that the subalkalic dacitic to rhyodacitic rocks have an adakitic composition with Na2o/K2o (1.8-3.16), high Sr (584-1750 ppm), Mg # = (0.18-0.57) and low Y (7-10 ppm), low Yb (0.65-1.29 ppm), and low HREE. Fractionated REE patterns, (Ce/Yb)N = 10-27, absence of negative Eu anomal, low content of Y, Nb, Ti, and high Sr/Y (74-265) and (Ce/Yb)N ratios suggest that the source was probably hydrous garnet-amphibolite or amphibole-eclogite, possibly generated during subduction of the Neo-Tethyan oceanic slab beneath the Central Iran microplate.The adakitic volcanism was followed by eruption of alkaline magmatism in this area. Slab melting occurred after cessation of subduction, possibility from the collision. Transtensional tectonics accompanied by a locally extensional stress regime account for magma genesis and ascent.

Keywords: Post-collision; Dacite; Adakite; Neo-Tethys; Iran

microcontinent. They include the Central Iranian Introduction Volcanic Belt (CIVB), Sanandaj-Sirjan metamorphic The Tethyan orogenic collage formed from collision zone and Zagros-folded-thrust belt (Fig. 1) [2-44-65]. of dispersed fragments of Gondwana with Eurasia [8- The CIVB contains intrusive and extrusive rocks of 33-44-53-62]. Within this context, three major tectonic Cretaceous -Quternary age that forms a belt with 50 Km elements with NW-SE trends are recognized in Iran due wide and 4 Km thick [6] that extends from NW to SE in to collision of Afro-Arabian continent and Iranian Iran. However, peak of magmatic activity is thought to

* Corresponding author, Tel.: 09368845513, Fax: +98(341)3222035, E-mail: [email protected]

223 Vol. 19 No. 3 Summer 2008 Ghadami et al. J. Sci. I. R. Iran

be Eocene age [3-23-67]. Geochemical studies indicate Eocene, Oligocen and volcano-sedimentary rocks of that the CIVB is generally composed of subduction- CIVB, (Fig. 3). In the south-west of the Khabr relate calc-alkaline rocks [8-24-34]. Alkaline rocks also subvolcanic domes intruded in to the ophiolitic rocks. are reported locally by [4], [29] and [45]. [4], proposed The ophiolitic rocks belong to Nain-Baft ophiolite. The a rift model to interpret the genesis of Eocene magmatic field study and the comparison of Figures 2 and 3, rocks in the CIVB. [6] argued that the onset of alkaline indicate that these andesitic and dacitic rocks were volcanism, which followed the calc-alkaline volcanism emplaced along faults and fractures. It is believe that the (6-5 Ma) in CIVB was due to sinking of the final broken ascent of acidic magma may be relate to the fault pieces of oceanic slab to a depth where alkaline melts activity in this area (Fig. 3). were generated. [25] suggested that post-suturing magmatic activity along the Sanandaj-Sirjan zone and CIVB can be attributed to slab break-off. In CIVB, area of Javazm, Dehaj and Khabr a geologic complexity and magmatic activity from calc- alkaline to alkaline presented. The diversity of magmatic types from calk-alkaline to alkaline indicate region of Javazm, Dehaj and Khabr to represent classical areas of young volcanism. The most intense eruptions were during the post collisional stage, which led to the formation of great volcanoes like Mosahim, Madvar, Aj Bala, Aj Pain and other volcanoes in this region. The great diversity of Neogene to Quaternary magmatic rocks, from andesitic, dacitic to rhyodacitic subvolcanic domes and extending for more than 150 km, are of interested due to their specific conditions of formation and spatial and temporal relation with other magmatic rocks. The dacitic volcanism of late Pliocene and Pleistocene in this area was followed by alkaline volcanism in Plio-Quaterner [33]. A conspicuous characteristic of this phase is the contemporaneous eruptions of mafic alkaline melts including melafoidites and alkali basalts [7-29]. The temporal and spatial Figure 1. Three main tectonic units of the Zagros orogenic relationship of dacitic calc-alkaline magmatism with belt [2-44]; □ Studied area. alkaline volcanism is also reported from different areas of Gondwana fragments and Eurasia plate collision zone [12-48-58]. The aims of this paper are (1) to present chemical characteristics of the dacitic to rhyodacitic magmatism in central Iran, (2) to suggest the conditions of their genesis, and (3) to discuss geodynamic environment in which they could have formed.

Material and Methods

2.1. Regional Setting The investigated areas are situated at the Central Iranian Volcanic Belt (CIVB), north west of Shahre- Babak City (Fig. 1-3). These regions are situated between Rafsanjan fault and Nain-Baft fault (Shahre- Figure 2. Tectonic map of the studied area that show by a Babak fault) (Fig. 2). In this area, numerous (n~50) square, studied area bounded by two main faults and fractures subvolcanic domes are intruded in to the volcanic [19,20].

224 Post-Collisional Plio-Pleistocene Adakitic Volcanism in Centeral Iranian Volcanic Belt: Geochemical and…

The Eocene volcanic and volcaniclastic rocks consist were chosen for major, trace and rare-earth elements of basalt, andesitic-basalt, brecciated volcaniclastic (REE) analysis. Samples for whole rock analysis were rocks and green tuffs. The oligocen dioritic plutons crushed and powdered in agate ball-mills. Major consist of porphyritic diorite, granodiorite and quartes- elements were determined by ME-ICP method. diorite rocks.The ophiolite rocks situated are east of the Inductively Coupled Plasma-Mass Spectorometry (ICP- Khabr and consist of basalt,gabbro,serpantine and MS) was employed for REE and trace element analysis pelagic sediment, and age of this rocks are Cretaceous for all of the samples. All of the analysis were and was emplaced before the Paleocene [19]. The age of determined at Actlabs laboratories (Canada). Represen- emplacement of subvolcanic domes are Plio-Pleistocene tative chemical analysis for major, trace and rare earth [19-20], but has been determined by [45], which yield elements are presented in Tables 1, 2 and 3. ages 7-17 Ma based on Amphiobol and Biotite on K/Ar dating methods. Field studies indicate that these domes 2.3.2. Analytical Results were emplaced along faults and fractures that developed The SiO2 of samples vary from 59 to 67.5 wt %. in shear zone with the Nain-Baf (Share Babak) fault in Using SiO2 vs. Zr/TiO2 diagram of [71] for south and Rasanjan fault in north of region (Fig. 2). classification of volcanic rocks, the studied samples plot Strike-slip tectonics accompanied by a transtensional in the fields of andesite and dacite-rhyodacite (Fig. 4). regime may also account for generation of adakitic and Also these rocks are metaluminous with Al2O3/ alkaline magmatisme in this area. Alkaline magmatisme (CaO+Na2O+K2O) ratios of 1.5-2.0. related to Plio-Quaterner [7-29]. Using SiO2 as a fractionation index, the samples display chemical variation and clear trends on Harker diagrams (Fig. 5). On variation diagram FeOt, MgO, 2.2. Petrographic Features TiO2, CaO, MnO and P2O5 display negative The porphyritic volcanic rocks consist of correlations, suggesting that these volcanic rocks intermediate to felsic suites whose composition varies experienced fractionation of apatite, hornblende and from hornblende-andesite, dacite and rhyodacite. plagioclase. The scattered of the samples belong to Dacitic and rhyodacite rocks are dominant and show present of phenocryst (fractional crystallization) or porphyritic texture with phenocrysts of plagioclase, assimilation of continental crust. The other oxides and hornblende and biotite. Plagioclase is ubiquitous elements (e.g. K2O, Na2O, Zr, Ba and etc.) display phenocryst (25-50 vol %) and contains inclusions of scatter trends. magnetite, amphibole and opaque. Andesites and dacites The K2O/Na2O ratios vary from 0.32 to 0.63 and contain large plagioclase crystals (3-5 mm) that usually samples are plot in the medium-potassium field and exhibit sieve textures and well defined zoning marked by concentric zones rich in/or devoid of glass and opaque inclusion. In some samples, plagioclase phenocrysts are mantled by a rim devoid of inclusions, whereas the core is rich in inclusions. Hornblende occurs as the main ferromagnesian phenocryst (up to 2 mm) in andesite and dacite and varies from green to brown in color. Hornblend often opassitezed and change to opaque and iron oxide. In some samples, accumulation of hornblende led to formation of glomeroporphyric texture. The groundmass is composed of plagioclase and hornblend as the main minerals, with apatite, biotite, quartz and iron oxides as accessory minerals.

2.3. Geochemical Characteristics

2.3.1. Analytical Methods About 200 samples of the dacitic rocks were collected from different domes. In order to correctly Figure 3. Geological map of area (Simplified from the characterize their chemical composition, 29 samples geological map of 1: 250000 Anar), [19].

225 Vol. 19 No. 3 Summer 2008 Ghadami et al. J. Sci. I. R. Iran

high potassium field (Fig. 6). In the AFM diagram of 0.2 [32], (Fig. 7).In diagram Na2O+K2O vs. SiO2 [32], all of the samples plot in subalkaline field (Fig. 8). In Y vs. 0.1

Zr diagram [37] the sample plote in the calc-alkaline MnO field (Fig. 12). The Mg # [MgO/(MgO+FeO)] of the samples ranges from 0.18 to 0.57 and contain high

0.0 concentration of Sr (584 to 1750 ppm) and low contents 60 62 64 66 68 70 of Y and Rb. The high contents of Sr, high ratios of SiO2 K2O/Na2O, Mg # (mean 0.42) and concentrations of Rb and Y indicate geochemical characteristics different 5 from typical volcanic rocks. 4 In the Y vs. Sr/Y diagram, all of the samples plot in 3 the field of adakite relative to typical arc-related calc- FeOT alkaline rocks defined by [15], (Fig. 9). In the (La/Yb)N 2 vs. YbN diagram all of the samples plot in the field of 1 adakite relative to classical island arc rocks defined by 60 62 64 66 68 70 [40], (Fig. 10). Figure 13 shows a spider diagram plot SiO2 for representative samples from different domes 0.15 normalized to primitive mantle composition [68]. All of the samples exhibit typical subduction-related 0.10 signatures: they are enriched in large ion lithophile MnO elements such as K, Rb and Ba and light REE relative to 0.05 HFSE and HREE and negative Nb anomalies. They

0.00 show significant positive anomalies for Sr, indicative of 60 62 64 66 68 70 either the absence of plagioclase fractionation or SiO2 retention of plagioclase in the residue. The adakites 7 exhibit Sr enrichment, in contrast to non-adakitic 6 dacities and rhyodacites which show negative anomalies 5 inspider diagrams. 4

Figure 14 displays the incompatible element patterns CaO 3 of representative samples normalized to average N- 2 MORB of [68]. LILE and LREE enrichment can result 1 0 from low degree partial melting of a MORB source. 60 62 64 66 68 70 Decoupling of Zr and Ti with similar bulk Kd,s and SiO2 greater depletion of Ti has been interprete to reflect a 0.7

0.6

0.5 80 0.4

Rhyolite TiO2 0.3 Com/Pan 0.2

70 Rhyodacite-Dacite 0.1 0.0 60 62 64 66 68 70 Trachyte SiO2 2 60 Andesite TrAn

SiO 3 Phonolite

50 Sub-Ab 2 Ab Bas-Trach-Neph MgO 1 40 .001 0.01 0.1 1 10 0 Zr/TiO2*0.0001 60 62 64 66 68 70 Si O2

Figure 4. Classification of volcanic rocks by Zr/TiO2 vs. SiO2 [71]. Studied samples compositions range from Andesite, to Figure 5. Variation diagrams for major and trace elements of dacite-rhyodacitic. samples ♦ Andesite, ● Dacite.

226 Post-Collisional Plio-Pleistocene Adakitic Volcanism in Centeral Iranian Volcanic Belt: Geochemical and…

4.0 Acidic residual phase in the source that fractionated Ti [47] or 3.5 Basic Ti-bearing phases [52]. The strong depletion of Y and 3.0 Yb corresponds to presence of restite garnet in the 2.5 High-K eclogitic source. The REE concentrations for samples of 2.0

K2O adakitic rocks from study area are plotted relative to 1.5 Medium-K chondrite in Figure 15. The ∑REE ranges from 61 to 1.0 173 ppm. The REE patterns for adakitic rocks from the 0.5 Low-K study area are similar, although the abundances are 0.0 50 55 60 65 variable. All the samples are enriched in LREE and SiO2 strongly fractionated in LREE and have a flat MREE to HREE pattern (LaN/YbN values of 11 to 38). The REE Figure 6. K2O vs. SiO2 for dacitic rocks of Central Volcanic Belt of Iran. Most of the samples plat in medium-k field and patterns for the rocks from the study area are linear with high-K[26], ♦ Andesite, ● Dacite. a small positive Eu anomaly, implying their cogenetic nature and derived from source regions that had similar FeOt relative concentrations of REE and similar mineralogy. Also the parallel nature of the REE patterns established that the residues had a big partition coefficient for these Tholeiitic elements, and consequently that they were grant. Low abundances of HREE in adakitic magma reflect retention of these elements in residual garnet in the partially melted subducted slab amphibol-eclogite.

Calc-Alkaline There are no cross-cutting REE patterns, suggesting that the studied magmatic suites are possibly related to and Na2O+K2O MgO most likely derived from the same initial melt. Figure 7. AFM diagram of [32]. All of the samples plot in Calc-Alkaline field, ♦ Andesite, ● Dacite. Results and Discussion

20 18 3.1. Discussion 16 14 The degree to which anatexis of subducted oceanic 12 crust has contributed to magmatism in convergent plate 10 margins has been a point of controversy for decades 8 Alkaline Na2O+K2O 6 [18]. As discussed by [26], arc magmas of basaltic 4 composition are regarded as products of mantle, not slab 2 Subalkaline anatexis, although some later workers continued to press 0 35 40 45 50 55 60 65 70 75 80 85 for slab anatexis in the production of arc basalts, SiO2 particulary those with high Al-contents [41]. Hydrated mantle peridotite as the principal source for arc basalts Figure 8. Na2O + K2O vs. SiO2 diagram [32], all of the samples plot in subalkaline field, ♦ Andesite, ● Dacite. is now firmly established [69], but genesis of intermediate and felsic are magmas remains controversial. The issue of slab anatexis as a globally important process was emphasized by [15] and [21] who demonstrated a connection between subduction of young oceanic crust and production of intermediate to felsic igneous rocks which bear the signature of a garnetiferous residuum. Such magmatic rocks are compositionally similar to Tertiary lavas on Adak Island in the Aleutian arc which were identified as products of slab melting by [35]. This petrologic family, termed Figure 9. Sr/Y vs. Y diagram [15] discriminating between "adakites", was described by [15] as high-alumina, adakitic and classical arc calc-alkaline compositions, intermediate to felsic volcanic rocks typically hosting ♦ Andesite, ● Dacite. phenocrysts of plagioclase, amphibole, mica and

227 Vol. 19 No. 3 Summer 2008 Ghadami et al. J. Sci. I. R. Iran

Table 1. Major and trace element contents of representative adakitic samples of CIVB

Sample 1-1 2-1 9-1 11-5 13-1 18-5 19-6 20-1 21-2 24-2 24-10 28-2 30-1 31-4 33-3 Rock type An An Da Da Da An Da Da Da Da Da Da Da An Da

SiO2 62.1 62.2 64 62.9 63.4 62.4 63.2 66.7 62.9 62.7 64.3 67.4 66.1 61.3 65.2

Al2O3 16.15 16.2 16.4 16.7 16.45 17.5 16.65 16.15 16.6 16.2 16.9 15.95 15.55 16.4 16.8

Fe2O3* 4.27 3.86 3.6 3.81 3.52 4.62 3.63 2.42 3.86 3.81 3.91 2.84 2.74 4.27 3.51 CaO 5.03 4.37 4.71 4.66 4.35 5.16 4.64 3.32 4.36 4.72 4.5 3.63 3.83 4.94 3.87 MgO 2.14 1.67 1.31 1.18 1.52 2.16 1.12 0.33 1.27 1.34 1.46 0.48 0.84 2.27 0.72

Na2O 3.76 4.76 5.03 5.22 5.1 5.08 5.17 4.73 4.96 4.91 5.15 4.6 4.54 5.03 4.63

K2O 1.19 2.02 1.73 1.86 2 1.93 1.86 2.5 1.99 1.91 1.91 2.49 2.38 1.93 2.36

Cr2O3 0.03 0.03 0.02 0.02 0.03 0.02 0.02 0.04 0.02 0.04 0.04 0.02 0.02 0.02 0.03

TiO2 0.4 0.48 0.44 0.43 0.42 0.55 0.46 0.43 0.53 0.45 0.46 0.38 0.35 0.57 0.45 MnO 0.1 0.06 0.04 0.07 0.06 0.08 0.06 0.02 0.04 0.05 0.06 0.03 0.03 0.07 0.06

P2O5 0.13 0.22 0.19 0.15 0.2 0.2 0.22 0.19 0.31 0.19 0.19 0.17 0.16 0.29 0.2 lOI 3.18 2.17 1.86 2.54 2.97 0.69 0.93 1.22 1.17 1.54 0.98 1.73 1.33 1.34 2.03 Total 99 98.2 99.5 99.8 100 100 98.1 98.2 98.2 98 100 99.9 98 98.7 100

Na2O/K2O 3.16 2.36 2.91 2.81 2.55 2.63 2.78 1.89 2.49 2.57 2.7 1.58 1.91 2.61 1.96 Mg# 0.47 0.46 0.42 0.38 0.46 0.48 0.46 0.21 0.39 0.41 0.42 0.25 0.38 0.51 0.29 Ba 395 596 561 687 676 640 647 657 700 632 627 635 629 713 642 Cr 210 240 140 90 190 150 160 330 130 280 310 130 130 130 180 Cs 1.33 0.93 0.71 0.67 0.85 1.44 1.09 2.27 0.82 1.8 1.36 2.13 1.96 1.19 2.88 Cu 21 38 40 23 42 54 25 44 35 49 43 45 40 43 43 Ga 19 20.6 20.5 20.5 21.3 21.6 21.1 21.9 20.8 21.3 21.1 20.7 20.3 20.6 20.6 Hf 2.4 3.1 3.1 3.2 3.1 3.1 3.2 4 3.4 3.1 3.2 3.6 3.6 3.4 3.8 Mo 6 5 4 2 3 3 4 10 3 5 9 3 3 3 3 Nb 2.5 4.7 4.5 5.8 5.3 5.6 5 6.3 7.2 5 5 5.4 5.3 9.5 6.2 Ni 17 26 16 14 23 42 17 23 29 25 23 14 12 30 16 Pb 8 14 12 12 15 13 12 16 15 13 14 16 19 13 17 Sr 584 950 913 1175 979 1040 1005 662 1225 942 945 633 585 1230 745 Rb 26.3 32.5 33.4 29.4 39.4 39.8 37.1 65.3 38 42.7 41.4 63.2 62.4 35.5 60.4 Ta 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.6 0.6 0.5 0.5 0.6 0.6 0.7 0.7

*Fe2O3 as total Fe2O3. Majer elements Wt %, trace elements and REE in ppm. An: Andesite, Da: Dacite-Rhyodacite.

(rarely) orthopyroxene, and lacking phenocrysts of (CIVB). Enrichment of LILE and depletion of HFSE clinopyroxene. Accessory grains of titanomagnetite, (Nb and Ti) and HREE are characteristic of subduction apatite, zircon and titanite were identified as common zone magmatism [15-16-39-40-70]. on the other hand but not ubiquitous. the high ratios of Na2O/K2O, high Sr, Mg #, Sr/Y and A complementary and broadly accepted chemical (Ce/Yb)N suggest an adakitic character for subduction- definition of adakites was sub-sequently provided by related magmatism [15-16-69-40]. [16]: adakites are high-silica (SiO2>56%), high-alumina The origin of adakites has been attributed to partial (Al2O3>15%), plagioclase and amphibole-bearing lavas melting of either subducted oceanic crust converted to with Na2O>3.5%, high Sr (>400ppm), low Y (<18ppm), eclogite and garnet amphibolite [15-21-35-38-39], or high Sr/Y (>40), low Yb (<1.9), and high La/Yb>20. underplating of basaltic magmas under thick continental Geochemically, it appears that subduction related crust [5]. components played a controlling role in the genesis of The strongly fractionated REE pattern and depletions the dacitic magmas in Central Volcanic Belt of Iran HREE and Y in adakites are possibly due to the

228 Post-Collisional Plio-Pleistocene Adakitic Volcanism in Centeral Iranian Volcanic Belt: Geochemical and…

Table 1. Continued

Sample 1-1 2-1 9-1 11-5 13-1 18-5 19-6 20-1 21-2 24-2 24-1 28-2 30-1 31-4 33-3 Rock type An An Da Da Da An Da Da Da Da Da Da Da An Da Th 2.5 4.86 3.96 5.66 4.87 4.2 4.16 6.94 11.35 4.18 4.16 6.63 6.63 7.16 7.15 U 1.13 1.46 1.33 1.67 1.67 1.42 1.42 2.67 2.91 1.47 1.47 2.65 2.33 2.09 2.71 V 106 93 80 62 74 101 86 74 99 84 90 60 56 97 67 W 4 12 5 6 11 3 4 9 3 15 8 6 5 6 13 Y 9.7 9.4 9 9.5 8.6 9.8 8.6 7.8 8.9 8.8 8.4 7.9 7.9 10.5 9.3 Zr 81 114 110 115 115 104 120 153 125 111 109 135 124 120 135 Ti 2400 2880 2640 2580 2520 3300 2760 2580 3180 2700 2760 2280 2100 3420 2700 P 567 960 829 654 873 873 960 829 1353 829 829 742 698 1266 873 Rb/Sr 0.05 0.03 0.04 0.03 0.04 0.04 0.04 0.1 0.03 0.05 0.04 0.1 0.11 0.03 0.08 Sr/Y 60.21 101 101 123.7 113.9 106.1 116.9 84.5 137.7 107 112 80.2 74 117 80 La 12 24.5 20.7 28.6 24.5 23.3 22 26.1 42.3 20.5 20.1 23.3 21.9 36 24.4 Ce 24.7 46.7 40.3 55.2 45.2 44.3 43.3 49.6 78.7 40.5 40 43.3 40.8 69.2 44.3 Pr 2.8 5.04 4.42 5.8 4.82 4.95 4.76 5.25 8.2 4.49 4.38 4.56 4.22 7.3 4.7 Nd 11.4 19.3 16.9 21.4 18.2 19.2 18.4 19 29.8 17.2 16.6 16.4 15.7 27 17.5 Sm 2.29 3.57 3.14 3.79 3.33 3.5 3.38 3.38 4.64 3.41 3.1 2.95 3 4.62 3.49 Eu 0.83 1.04 1 1.08 0.98 1.08 0.98 0.96 1.38 0.92 0.96 0.89 0.83 1.32 1.06 Gd 2.23 2.96 2.79 3.36 2.75 3.22 3 2.66 3.91 2.92 2.73 2.58 2.71 4.11 3.15 Tb 0.33 0.39 0.37 0.42 0.35 0.41 0.37 0.33 0.48 0.35 0.37 0.35 0.34 0.5 0.42 Dy 1.84 1.85 1.85 1.98 1.69 2.05 1.17 1.6 1.99 1.76 1.81 1.57 1.83 2.32 2.01 Ho 0.34 0.34 0.35 0.35 0.28 0.36 0.33 0.27 0.33 0.32 0.33 0.28 0.32 0.4 0.38 Er 0.98 0.97 0.97 1.08 0.93 1.02 0.87 0.8 0.95 0.94 0.98 0.84 0.88 1.17 1.08 Tm 0.12 0.12 0.13 0.13 0.1 0.13 0.11 0.09 0.1 0.12 0.12 0.11 0.12 0.14 0.14 Yb 1.01 0.84 0.89 0.88 0.73 0.8 0.76 0.65 0.75 0.84 0.81 0.8 0.81 0.91 0.99 Lu 0.14 0.12 0.12 0.13 0.1 0.12 0.1 0.1 0.09 0.09 0.11 0.1 0.12 0.15 0.15

(La/Yb)N 15.65 19.62 15.67 22.26 23 19.84 19.97 27.35 38.4 16.41 17.09 19.81 18.36 27.25 16.63

(Ce/Yb)N 6.45 14.35 11.69 16.49 16.28 14.5 15 19.9 27.4 14.44 13 14.1 13.31 20 11.6 ∑ REE 61 107.7 93.93 124.2 103.5 104.5 100.1 110 173 94.4 92.4 98 93.5 155 103.7

150 1.0

K2O/Na2O LaN/YbN 120 0.8

90 0.6 FC Adakites 60 0.4 Classical Island ARC 30 0.2 AFC

0 0.0 0 10 20 30 0.0 0.2 0.4 0.6 0.8 1.0

YbYbnN Rb/ZrRbTZr

Figure 10. (La/Yb)N vs. YbN diagram [39] discriminating Figure 11. Diagram of K2O/Na2O vs. Rb/Zr shows fractional between adakitic and classical arc calc- alkaline compositions, crystallization (FC) and assimilation fractional crystallization ♦ Andesite, ● Dacite. (AFC) trends [22], ♦ Andesite, ● Dacite.

229 Vol. 19 No. 3 Summer 2008 Ghadami et al. J. Sci. I. R. Iran

Table 2. Major and trace element contents of representative adakitic samples of CIVB

Sample 35-d 38-1 46-4 48-4 50-2 52-3 54-3 55-2 58-1 60-2 63-4 64-1 65-2 68-1a Rock type Da Da An An Da Da An An Da Da Da An An Da

SiO2 63.7 63.2 65.3 59.8 66.3 63.5 62.5 61.8 64.1 63.4 63.2 62.3 60.2 64

Al2O3 16.8 16.55 14.95 16.2 17.65 16.4 17.2 16.05 16.4 16.2 16.4 16.05 16.35 15.8

Fe2O3* 3.74 3.88 2.77 4.79 2.14 3.95 4.14 4.17 3.94 3.49 4.03 3.87 3.66 3.36 CaO 4.2 4.51 6.1 6.1 2.92 4.52 4.34 5.04 3.97 4.24 4.74 4.33 4.64 4.36 MgO 1.26 1.6 1.13 3.22 0.24 1.84 1.48 2.07 1.52 1.58 1.73 1.8 2.42 1.33

Na2O 4.76 4.92 4.52 5.05 4.35 4.7 4.69 4.44 4.76 4.58 4.68 4.53 4.35 4.9

K2O 2.17 2.1 1.92 1.61 2.47 2.22 2.45 2.78 2.81 2.53 2.59 2.46 2.47 1.66

Cr2O3 0.02 0.03 0.02 0.03 0.02 0.04 0.02 0.02 0.01 0.02 0.03 0.02 0.02 0.02

TiO2 0.44 0.47 0.32 0.61 0.42 0.49 0.54 0.56 0.52 0.42 0.5 0.48 0.06 0.05 MnO 0.06 0.06 0.05 0.08 0.01 0.07 0.05 0.07 0.06 0.05 0.06 0.07 0.06 0.05

P2O5 0.23 0.24 0.15 0.27 0.15 0.24 0.26 0.26 0.23 0.2 0.22 0.19 0.2 0.17 lOI 2.4 2.44 5.48 2.16 3.15 1.74 2.13 2.6 1.22 2.3 1.72 1.93 2.17 1.29 Total 100 100 100 100 100 99.9 100 100 99.8 99.2 100 98.2 98.2 98.1

Na2O/K2O 2.19 2.34 2.35 3.14 1.76 2.12 1.91 1.6 1.69 1.81 1.81 1.84 1.76 2.95 Mg# 0.4 0.45 0.44 0.57 0.18 0.48 0.41 0.49 0.43 0.47 0.46 0.48 0.5 0.4 Ba 781 713 653 659 1025 695 764 645 654 605 615 585 826 493 Cr 120 240 150 180 120 250 120 180 130 190 250 160 180 170 Cs 1.9 1.72 1.72 1.13 4.99 2 1.68 1.34 2 1.48 1.47 1.28 2.48 0.64 Cu 29 39 39 48 20 36 44 38 17 23 40 46 59 53 Ga 20.3 20.7 19.1 20.5 20.3 20.4 21.4 19.4 19.7 19 20.1 20.5 19.5 21.2 Hf 3.3 3.8 3.5 3.5 3.8 3.6 3.4 3.6 3.7 3.4 3.4 3.3 2.8 3.6 Mo 4 5 4 3 3 4 3 2 2 5 4 4 2 5 Nb 5.8 6.3 4.4 6.7 7.8 7.6 6.9 11.1 10.3 8.2 8 7.6 4.9 4 Ni 17 30 16 59 15 26 11 33 20 19 29 26 36 15 Pb 13 13 12 11 17 15 17 14 14 13 11 15 12 11 Sr 1125 1080 1750 1385 955 873 975 827 862 886 905 873 832 820 Rb 39.9 40.6 41.8 33.6 66.2 43.8 62 60.4 66.8 56.5 54.7 52.9 58.7 26.5 Ta 0.5 0.6 0.6 0.7 0.7 0.7 0.6 0.8 0.7 0.6 0.6 0.6 0.4 0.3

*Fe2O3 as total Fe2O3. Majer elements Wt %, trace elements and REE in ppm. An: Andesite, Da: Dacite-Rhyodacite. presence of garnet +/− amphibole in melt residue. Their in experimentally produced melts of amphibolite or high Sr and low Nb, Ta, and Ti contents are thought to eclogite[50- 61]. [60]and [73] attributed this enrichment be due to absence of plagioclase and presence of Fe-Ti to the interaction of adakitic magma with the mantle oxides in the residue [39]. While geochemical data for during ascent. Experimental work by [51] show that igneous rocks compiled by [15] indicate a relationship small amounts of adakitic melt are entirely consumed in between subducted oceanic crust and adakite genesis, reaction with mantle peridotite to from metasomatised adakite occurrences in different tectonic environment zones as has been proposed by [59] and [61]. lad [42] to propose that slab melting even of old oceanic On the other hand, when the ratio of melt/peridotite crust is also possible during: 1- The initiation of reaches 2:1, a portion of melt not consumed in the subduction [55-56]. 2- Fast and oblique subduction [35- reaction becomes Mg-enriched and preserves its trace- 46]. 3- Termination of subduction [49-57]. element geochemical characteristics such as high Sr/Y The high Mg and Cr content of most adakites are not and (Ce/Yb)N ratios. consistent with the low concentration of these elements The highly enriched N-MORB normalized

230 Post-Collisional Plio-Pleistocene Adakitic Volcanism in Centeral Iranian Volcanic Belt: Geochemical and…

Table 2. Continued

Sample 35-d 38-1 46-4 48-4 50-2 52-3 54-3 55-2 58-1 60-2 63-4 64-1 65-2 68-1a Rock type Da Da An An Da Da An An Da Da Da An An Da Th 5.24 5.44 6.94 4.66 3.85 7.71 4.77 7.14 7.56 7.11 6.58 6.45 8.15 4.1 U 1.86 1.76 2.15 1.45 3.96 2.03 1.73 2.31 2.41 2.23 2.08 1.96 2.97 1.22 V 78 78 55 109 67 79 85 89 100 76 84 86 110 62 W 4 11 9 6 7 13 8 7 7 7 11 4 9 5 Y 9.4 8.8 6.6 10.1 14.1 9.1 9.3 9.9 8.6 8.4 8.7 8.7 9 7.4 Zr 111 114 113 109 127 125 125 138 132 122 123 122 102 133 Ti 2640 2820 1920 3660 2520 2940 3240 3360 3120 2520 3000 2880 2820 2400 P 1004 742 654 1178 654 1047 1135 1135 1004 873 960 829 873 742 Rb/Sr 0.04 0.04 0.02 0.02 0.07 0.05 0.06 0.07 0.08 0.06 0.06 0.06 0.07 0.03 Sr/Y 119 122 165 137 67.7 95.9 105 83.6 100 105 104 100 92.5 109 La 26.3 27.4 20.1 25.2 30.2 27.2 24.7 32.6 26.4 25.3 25 25.2 23.4 25.2 Ce 49.6 49.9 35.7 50.5 57.7 49 46.2 51.8 48 46.5 48.2 47.6 42.6 43.3 Pr 5.37 5.38 3.77 5.58 6.17 5.2 5.17 5.85 5.36 5.14 5.5 5.44 4.82 5.06 Nd 19.9 20.1 13.8 21.5 24.3 19.3 19.1 22.1 20 19.6 20.4 20.6 18.2 19.3 Sm 3.51 3.48 2.67 4.2 4.86 3.44 3.36 3.73 3.37 3.24 3.45 3.46 3.2 0.93 Eu 1.08 1.17 0.88 1.4 1.47 1.07 1.12 0.95 0.92 0.91 0.94 0.99 0.96 0.93 Gd 3.32 3.26 2.69 3.93 4.56 3.03 3.3 3.14 2.87 2.8 2.95 2.92 2.84 2.84 Tb 0.42 0.44 0.31 0.5 0.62 0.41 0.39 0.41 0.36 0.33 0.34 0.37 0.35 0.33 Dy 1.97 2.09 1.63 2.43 2.96 1.89 2.16 1.89 1.72 1.67 1.74 1.81 1.88 1.51 Ho 0.37 0.35 0.28 0.44 0.56 0.33 0.36 0.34 0.29 0.27 0.31 0.3 0.32 0.27 Er 1.08 1.03 0.83 1.37 1.54 1.06 0.99 1.02 0.91 0.85 0.92 0.87 0.97 0.7 Tm 0.15 0.14 0.13 0.2 0.21 0.14 0.13 0.14 0.11 0.12 0.11 0.12 0.12 0.09 Yb 0.95 0.93 0.81 1.29 1.37 0.85 0.82 0.85 0.71 0.73 0.75 0.71 0.84 0.63 Lu 0.15 0.13 0.14 0.22 0.21 0.15 0.11 0.13 0.12 0.09 0.1 0.11 0.12 0.07

(La/Yb)N 18.84 20.16 17.09 11.27 14.95 21.79 20.39 26.12 25.68 23.76 3.48 24.5 18.74 27.44

(Ce/Yb)N 13.63 14 11.66 10.15 10.96 15.26 16.64 15.91 17.89 16.76 17.31 17.75 13.09 18.08 ∑ REE 114.2 115.8 83.74 118.7 136.7 113.7 107.9 124.9 111.1 107.5 110.7 110.5 100.3 103.6

80 300

Tholeiitic 100 60

Y 40 Transitional 10

20 Calc-alkaline Sample/Primitive Mantle

0 1 0 50 100 150 200 250 Ba Th Nb La Sr P Zr Ti Yb Zr Rb Ta Ce Nd Sm Hf Y

Figure 12. Diagram of Zr vs.Y after [36] all samples plot in Figure 13. Primitive mantle-normalized incompatible trace the calc-alkaline field, ♦ Andesite, ● Dacite. element diagram for all samples [68] ♦ Andesite, ● Dacite.

231 Vol. 19 No. 3 Summer 2008 Ghadami et al. J. Sci. I. R. Iran

abundance patterns of trace elements and REE pattern Zagros-Oman an offshore intra-oceanic are [9], while a for adakitic dacitic and rhyodacites of CIVB (area of vast area of oceanic lithosphere still existed to the north Javazm, Khabr and Dehaj) suggest the existence of of Zagros [17] yet to be subducted underneath central garnet as a residue in the source. The enrichment of Sr Iran during the Tertiary.An alternative idea is that and the absence of negative Eu anomalies indicate that continental collision along the Zagros suture occurred in the residual source was pelagioclase free. The Nb and Ti the Miocene [8-63-64]. Paleoceanographic constraints are strongly depleted in the studied samples, which derived from carbon and oxygen isotopic date indicate suggest that the source also has residual rutile and that Neo-Tethys had a connection with the northern amphibole and thus was most probably hydrous garnet- Indian Ocean until 14Ma [72]. This factor supports the amphibolite or amphibole-eclogite. This garnet-bearing Miocene reconstruction of Neo-Tethys by [63] and is source implies that there are at least two possibilities for independent of regional geological evidence. Existence generation of adakitic rocks in Central Iran: of widespread shallow marine and limited deep-marine 1. Partial melting of thickened lower crust and Paleocene to Miocene sediment in Zagros sub-zones is 2. or melting of subducted oceanic slab of Neo- consistent with the south arm of the Tethys remaining tethys. open in to Miocene [44]. Opening of the Red Sea and It is expected that crustal thickening caused by the Gulf of Aden resulted in rotation of the Arabian Arabian-Asian continental collision would result in plate with respect to Africa (Nubia ano Somalia) since transformation of basaltic lower crust in to garnet- 30 Ma [11-28-30]. This plate movement was respon- amphibolite or amphibole-eclogite. However, such sible for oblique convergence between the Arabian plate deeper crustal materials have not been observed nor and central Iran and final closure of Neo-Tethys. reported as xenoliths from the studied area. Moreover, according to the Moho depth map of [14] the crustal 200 thickness of the area ranges from 48 to 50 Km. A 100 seismic refraction profile through Sar Cheshmeh [27], however, gave crustal thicknesses of only 30 to 40 Km for the CIVB in Kerman province, which is not an adequate depth for conversion of basaltic lower crust in 10 to garnet- amphibolite or amphibole-eclogite. The other candidate hydrous amphibole-eclogite or 1

garnet-amphibolite, which could melt to generate Sample/N-Type MORB adakitic magmas in central Iran, is subducted Neo- Tethyan oceanic slab. The andesitic, dacitic and 0.2 Ba Th Nb La Sr P Zr Ti Yb rhyodacitic of study area show high Na2O/K2O, high Sr, Rb Ta Ce Nd Sm Hf Y low Y, strongly high REE depletion and high LREE. [46-60-61] believe that such compositional behavior of Figure 14. MORB-normalized incompatible trace element this rocks is consistent with their generation by melting diagram for all samples [68] ♦ Andesite, ● Dacite. of subducting oceanic lithosphere. The values of radiogenic Sr (0.704273 to 0.705668) and έNd (+ 1.3 to +4.1) for this rocks from [45] respectively indicate that 200 pelagic sediment could not have been involved in the 100 genesis of the andesitic and dacitic rocks. Partial melting of an amphibole-eclogite sourse would generate melts that have high Sr/Y, high LREE, but low Y and low HREE [15-16-39-40]. 10 Controversy exists in the literature about the timing of the closure of Neo-Tethyan along the Zagros suture. Sample/Chondrite Some authors infer a late cretaceous age for continental collision [2-7]. A late Cretaceous age for continent- 1 La Pr Eu Tb Ho Tm Lu continent collision comes from the timing of the Ce Nd Sm Gd Dy Er Yb ophiolite emplacement, i.e. age of the youngest pelagic fossils involved in the Zagros ophiolites. However, this Figure 15. Chondrite-normalized REE pattern of age has been shown to merely reflect the timing of representative dacitic samples of Central Iranian Volcanic Belt ophiolite abduction due to collision of passive margin of [68] ♦ Andesite, ● Dacite.

232 Post-Collisional Plio-Pleistocene Adakitic Volcanism in Centeral Iranian Volcanic Belt: Geochemical and…

Petrological studied carried out in this area and (4) The temporal and spatial adakitic rocks and adjacent area i.e. Mosahim, Madvar, Aj Bala, and Aj alkaline volcanic rocks in the studied area can be Pain indicate that post collisional magmas exhibit attributed to slab break off and melting of detached slab various geochemical enrichment signatures. The and metasomatised mantle. significant character of post-colllisional magmatism in (5) Volcanism along the dextral stike-slip faults is this area indicates the progressive evolution of mainly related to extension, followed by strike-slip magmatic products from subalcalin to alkaline toctonics in the context of regional tension. composition. A conspicuous characteristic of alkaline (6) The variations in K2O, LILE and HFSE contents phase is the contemporaneous of mafic alkaline melts in comparison with modern adakites can be attributed to including melafoidites and alkali-basalts [29]. fractionation and crystallization processes. Onset of post-collisional magmatism in the late Plio- Pleistocene in this region will adakitic geochemical signatures, indicate the role of slab melting after Acknowledgments cessation of subduction. The temporal and spatial This research is based on field and laboratory studies relationship of the studied adakitic rocks may be out at the University of Shahid Bahonare Kerman.I wish attributed to slab roll-back and possibly break-off to acknowledge the kind support of the staff of this subducted Neo-Tethyan oceanic lithosphere beneath the University. Central Iranian continental microplate. Slab break-off may have led to thermal perturbation resulting in melting of detached slab and metasomatism of the References mantle in Central Iran during the post-collisional event. 1. Ahmad, T., Posht Kuhi, M. 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